By fizzysid (510 – 1·15) on 2012/10/13 (Patch 5.0.5) Report
I can confirm that this achievement can be earned by faction-changing a lvl 90.
I switched back to horde today on my level 90 priest after a brief jaunt to alliance (I wanted to try MoP from both sides). The faction transfer processed, I chose my new race, logged in, and to my surprise, an achievement popped up straight away.
Just thought it would be worth mentioning!
Edit: You do not have to have two level 90s either. If you hit 90 on a horde toon (no lvl 90 alts) then switch it to alliance, you'll get the achievement. If you already have the achievement, you will not lose it by faction changing. The achievement is account-bound.